<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    div{
      height: 40px;
      margin: 20px;
    }
    #one{
      border: 1px solid green;
    }
    #two{
      border: 1px solid red;
    }
    #three{
      border: 1px solid deeppink;
    }
  </style>
</head>
<body>
  <div>
    <div id="one">
      我是第一层
      <div id="two">
        我是第二层
        <div id="three">
          我是第三层
        </div>
      </div>
    </div>
  </div>
</body>
</html>